In general, in order to improve wear resistance, cemented carbide used for cutting tools is used after a hard coating layer is formed on the surface thereof, and the hard coating is formed through a chemical vapor deposition method (hereinafter referred to as “CVD”) or a physical vapor deposition method (hereinafter referred to as “PVD”).
Meanwhile, during high-speed processing of a high-hardness material, a cutting edge of a cutting tool is exposed to a high-temperature environment of approximately 1000° C., and the cutting edge not only wears down due to friction and oxidation caused by contacting a workpiece, but also is subjected to mechanical shock such as a jerk. Therefore, the cutting tools are essentially required to have toughness together with appropriate wear resistance.
In general, a hard coating is composed of a single-layer or multi-layer non-oxide-based thin film (e.g., TiN, TiC, or TiCN), an oxide-based thin film having excellent oxidation resistance (e.g., Al2O3), or a mixed layer thereof. Examples of the non-oxide-based thin film includes carbides, nitrides, or carbonitrides of metal elements of Group IV, V, and VI in the periodic table, such as TiN, TiC or TiCN, and the examples of the oxide-based thin film representatively includes α-Al2O3 or K—Al2O3.
Meanwhile, a main disadvantage of the non-oxide-based thin film, such as carbides, nitrides, or carbonitrides of metal elements of Group IV, V, and VI in the periodic table, is inferior oxidation resistance, and such a problem is mainly being solved such that an oxide thin film having excellent oxidation resistance such as alumina (Al2O3) is laminated on a non-oxide thin film and is then subjected to multilayer coating.
However, since a thin film in which non-oxide thin films and oxide thin films are formed in multiple layers has inferior adhesion between the thin films, the mechanical strength tends to be unstable between the thin films during a cutting process in which a high-temperature environment is formed, and particularly, during the processing of workpieces having high stickiness (toughness) inherent in materials thereof, such as bearing steel, nickel-chromium steel, or cold forging steel, the adhesion between the non-oxide thin films and the oxide thin films are further demanded.
Among the oxide-based thin films, κ-Al2O3 has a merit of having excellent adhesion with a non-oxide-based thin film and being formed in a relatively low temperature, but has a limitation in that a phase transformation, in which a κ-phase is changed into an α-phase, may occur due to a high-temperature caused by cutting, and this phase transformation causes an approximately 6-8% of volume contraction and cracks, and thereby causes a phenomenon of delamination of Al2O3 thin film.
Compared to this, since α-Al2O3 is a phase stable in high-temperatures, α-Al2O3 has a merit of exhibiting excellent wear resistance without causing a phase transformation during a cutting process, but has a limitation in that in order to directly coating a non-oxide-based thin film with —Al2O3, a high temperature of approximately 1040° C. is required, and adhesion between the α-Al2O3 formed in this case and a non-oxide-based thin film located under the α-Al2O3 is decreased.
To solve the limitation, a method is used, in which an oxide layer such as TiCNO or TiCO having a similar composition to a TiCN layer or the like located under a non-oxide-based thin film is formed as a bonding layer on the non-oxide-based thin film, and then, an α-Al2O3 layer is formed on the bonding layer.
Through such a bonding layer, delamination resistance of the α-Al2O3 layer may be somewhat improved, but there is still a limitation of exhibiting an insufficient adhesion strength.
Patent documents, such as U.S. Pat. No. 7,993,742 and International Patent Publication No. WO2014/198881, disclose a feature in which the physical properties of a coating film are improved by controlling the grain size and the crystal growth direction of MT-TiCN, or the adhesion of a coating film is improved by controlling the shape and the crystal growth direction of an α-Al2O3 layer. However, there still remains a room for improvement.